1. Field of the Invention
The present invention relates to an organic light-emitting device (also referred to as “organic electroluminescence device” or “organic EL device”) which includes a novel metal complex compound for a light-emitting device; and is used in, for example, a surface light source or a flat panel display.
2. Description of the Related Art
An organic electroluminescence device has been recently expected to find applications in next-generation full-color displays because of its characteristics of a low driving voltage, high responsiveness, high efficiency, and high visibility. In association with the recent expectation, an applied research for forming an organic electroluminescence device that can be practically used, including the development of a material for such device, has been vigorously conducted.
As detailed in Macromol. Symp. 125, 1 to 48 (1997), an organic electroluminescence device is generally of such a structure that organic layers including a fluorescent light-emitting layer are formed between two electrodes as upper and lower electrodes formed on a transparent substrate.
Recently, studies have been made on a device using not only conventional light emission utilizing fluorescence at a time of transition from a singlet exciton to a ground state, but also phosphorescence emission through a triplet exciton which is typified by “Improved energy transfer in electrophosphorescent device (D. F. O'Brien, et al., Applied Physics Letters Vol. 74, No. 3, p. 442 (1999))” and “Very high-efficiency green organic light-emitting devices based on electrophosphorescence (M. A. Baldo, et al., Applied Physics Letters Vol. 75, No. 1, p. 4 (1999))”. In each of those documents, an organic layer having a four-layer structure is mainly used. The structure includes a hole transport layer, a light-emitting layer, an exciton diffusion blocking layer, and an electron transport layer in the stated order from an anode side. A phosphorescent light-emitting material contained in the above organic layers is a phosphorescent metal complex compound having Pt or Ir as its central metal, and Ir(ppy)3 (ppy: phenylpyridine) is a representative example of this compound. A host such as a carbazole- or triazole-based compound is doped with a low concentration (about several percent) of this compound.
Meanwhile, a research on a polymer type organic electroluminescence device useful for large screen size and cost reduction (no use of mask) has also been actively advanced. A polymeric material to be used in the polymer type organic electroluminescence device cannot be vapor-deposited (sublimated), but is soluble in an organic solvent or the like, and therefore an application method such as a spin coating method, a printing method, or an ink jet method is adapted for the material. A shadow mask is indispensable to patterning by a vapor deposition (sublimation) method, and therefore the patterning involves a large problem in that the utilization efficiency of a material is about several percent. On the other hand, the application method has a high advantage that the utilization efficiency of a material reaches 100% because the application method requires no mask.
Researches on use of a phosphorescent metal complex compound even in such polymer type organic electroluminescence device have also been conducted. Most of the researches each relate to a device having a light-emitting layer containing, as a host, a carrier transport polymer (for example, polyvinyl carbazole) doped with a low concentration (about several percent) of a phosphorescent metal complex compound (for example, Ir(ppy)3) (Nature, 347, 539 (1990)).
In addition, a group of University of Cambridge (Journal of the American Chemical Society, 126 (22), 7041 (2004)) has reported that Ir(ppy)3 as a phosphorescent metal complex compound is formed into a polymer. Specifically, the document describes a device using, in its light-emitting layer, a phosphorescent metal complex compound having a ligand obtained by introducing polyfluorene (having 40 or less repeating units) into a phenyl group of phenylpyridine (ppy) described above. However, the external quantum efficiency of the device is as low as 1.5% at maximum.
In addition, a group of NHK (Applied Physics Letters, 86, 103507 (2005)) has reported a material of a type which is obtained by bonding Ir(ppy)2acac (ppy: phenylpyridine, acac: acetylacetone) as a phosphorescent metal complex compound to a side chain of polyethylene. The concept of the above-mentioned material is different from that of the present invention, that is, “phosphorescent metal complex compound having no molecular weight distribution and a molecular weight which can be defined”. However, an organic electroluminescence device using this material shows an external quantum efficiency of 11.8% as a relatively high efficiency.
Known examples of a mechanism by which the emission efficiency of such polymer type organic electroluminescence device as described above is controlled include energy transfer from the singlet or triplet excited state of a polymer host to a phosphorescent metal complex compound, and direct excitation on the phosphorescent metal complex compound. Therefore, one means for causing the phosphorescent metal complex compound to efficiently emit light is to satisfy the following conditions: the singlet or triplet energy level of the host is higher than that of the phosphorescent metal complex compound, and the band gap of the host is large. Further, for example, the following three points are extremely important factors for causing the compound to more efficiently emit light. That is, a first point is that the solubility of the host and the phosphorescent metal complex compound in a solvent is excellent. A second point is that phosphorescent metal complex compound molecules do not agglomerate or associate (static quenching) in a solution or a film. A third point is that collision between excited molecules and other molecules (dynamic quenching) is suppressed. In addition, a polymeric material has a molecular weight distribution, and the molecules of the material are not formed of a single component. Therefore, when the material is formed into a film, the material inevitably involves a problem that a defect or an impurity is observed in the film. This problem largely affects the long-term stability (life-time of the device) of the device, and is one cause for the fact that the life-time of a polymer type device is hardly comparable to that of a low weight molecular type device.